The invention relates to an arrangement for injecting a reducing agent into an exhaust line of a combustion engine.
Today, many combustion engines and the majority of diesel engines are provided with a turbo unit. A turbo unit comprises an exhaust-powered turbine driving a compressor which compresses air in an inlet line to the engine. Air is thus supplied at a positive pressure to the engine's combustion spaces. A large amount of air and fuel can thus be supplied to and burnt in the engine. An engine with turbo can therefore produce a significantly higher power output than a similar engine with no turbo. The amount of power which the turbine can extract from the exhaust gases in order to drive the compressor is substantially related to the pressure drop of the exhaust gases across the turbine.
One way of reducing emissions of nitrogen oxides from diesel engines is to use a technique called SCR (selective catalytic reduction). This involves a reducing agent in the form of a urea solution being supplied in a specific dose to the exhaust gases in the exhaust line of a diesel engine. When the urea solution is sprayed into the exhaust line, the resulting finely divided solution becomes vaporised by contact with the hot exhaust gases so that ammonia is formed. The mixture of ammonia and exhaust gases is then led through an SCR catalyst in which the nitrogen in the nitrogen oxides in the exhaust gases reacts with the nitrogen in the ammonia to form nitrogen gas. The oxygen in the nitrogen oxides reacts with the hydrogen in the ammonia to form water. The nitrogen oxides in the exhaust gases are thus reduced in the catalyst to nitrogen gas and water vapour. With correct dosage of urea, the engine's emissions of nitrogen oxides can be greatly reduced.
Vehicles powered by a so-called turbo compound engine have an extra turbine in the exhaust line downstream of the turbine of the turbo unit. The extra turbine likewise extracts energy from the exhaust gases. This energy is used to provide the vehicle with extra power. The turbine transmits the energy extracted to a driveshaft of the engine via a transmission which may comprise a viscoclutch. A turbo compound engine therefore has a somewhat higher efficiency than a similar turbo engine. The exhaust line of a turbo compound engine thus comprises two turbines. The temperature and pressure of the exhaust gases are thus reduced in two stages in the exhaust line. In certain operating situations, the temperature of the exhaust gases downstream of the second turbine is not high enough to vaporise the urea solution injected into the exhaust line.